Standing back from your questions....you seem to be trying to cater for, ...as you say hundreds of backers. If they are wise enough to put funds into your project....they are wise enough to understand that Apples sells your books if you put into the store as a paid book...presumably, they hope to make a return on the investment....and should be prepared to actually buy a copy do they in turn can show it around and induce others to buy! That would solve your distribution problem.
If you distribute your book to backers free, if/when you update it...you send it out again, telling everybody to delete the old copy from all devices.
All your other thoughts about free periods of purchase etc.....are not possible. If you consider my approach....your problems are solved.
you can change the price on your book up or down as you wish and set the periods the price applies.
A more messy to meet your needs, is maybe open a free book account.....upload it...tell your backers they have 24 hours to download before it goes offline. The you simply go to iTunesConnect>Manage your Books, select the book, go to territory rights and pricing and remove the book from all stores.
Good luck with the book. You obviously need to sell in large numbers for backers to get and return on investments.....